Electron discharge device



ELECTRON DISCHARGE .DEVI CE Original Filed July 31, 1957 INVENTORS JOHNWILL/AM STRANGE AND H WILLIAM HORACE CONNELL wk I A TTORIVEY.

Patented Dec. 31, 1940 T OFFICE John William Strange, L

ondon, and William Horace Connell, Hillingdon, England, assignors to HElectric '& Musical Industries Limited, Hayes,

. l lngland, a company of Great Britain '(iriginahapplication July 31,1937, Serial No.

this application October 3,178. In Great Britain 1 SClainis. (01.250-275) I r due to the fact that both the mask and the pic- Thisinvention relates tocathode ray tubes such as are employedfor thereconstitution of a picture fortelevision purposes. w y

.This case is a division of my copending application Serial No..156,610, filed July 31, 1937, which issued as Patent No. 2,158,640 onMay 16; 1939. v

Usually, the envelope of a tub-e of this kind is made from relativelythin glass, and in order to mask the unused portion of the fluorescentscreen and for. the purpose of defining the picture area theoutersurface of the. tube is provided with a mask painted or otherwiseapplied thereto; .In somecases. the plctureproduced by the fluores- 1cent screen is projected .on to a screen and since withthisarrangementit is necessary to employ a lens for projecting the image onto the screen, it is; desirable that the end wall of the tube on whichthe fluorescent screen is formed should be flat, otherwise difiiculties;are encountered when projecting the image..-

In the usual time of cathode vray tube the end wall on which thefluorescent screen is iormedis convextfor the purpose of withstanding.the high degree of vacuum necessary for the proper operation of thetube, but in tubes in which the reconstituted picture is projected ontoa screen and it is necessary to make. the end wall fiat it isconsequently essential thatthe Walls of the tube be substantiallythickened in order to withstand the required degree of vacuum. Owing tothe thickeningof theend wall it is found that if the mask isapplied tothe outer surface of such wall the outline of the mask when projectedresults in an ill-defined or feathery edge bounding the edges of theprojected image instead ofa desirable sharply defined boundary. Wherethe wall of thetube is of relatively thinglass this difficulty does notarise since it is usual to view the image directly from the end of thetube, but in the so-called projection type of cathode ray tube since itis necessaryto make the end Wall thicker so that such end wall can bemaintained flat the difficulty above-mentioned is encountered.

It is the chief object of the present invention to provide an improvedconstruction of cathode ray tube of the projection type in which thedifliculty above-mentioned is overcome. According to the. invention, acathode ray tube of the projection type is provided in which the mask isapplied on the same side and to the same surface as that on which thefluorescent screen is formed. In such a construction the projected imagewill be bounded by a sharply defined edge corresponding in outline tothe shape of the said mask. This is ture can be sharply focusedsimultaneously by a suitable lens system such as will beemployed for thepurpose in view.

As illustrated in the drawing, schematically one embodiment of myinvention is as follows:

A cathode ray tube I having a planar end wall 5 has mounted within thetube an electron beam forming system 3. .A deflecting coil system II and13 are provided for deflectingthe beam in accordance with thepredetermined pattern over the end wall 5. Internally of the envelope,there is deposited luminescent screen material 1 which is adapted toemit light under the bombardment of the projected cathode ray beam. Themask 9 is likewise mounted internallyof the tube on the end wall 5 todefine the boundary of the luminescent screen. The thickness of the mask9 is substantially equal to the depth of the luminescent materialdeposited. An optical system I5 is provided for focusing the luminescentimage produced upon a screen I l. v

The mask in most cases must be formed of such a material or must be ofsuch a nature that it can resist the temperatures encountered during theprocess of evacuation such temperatures usually being in theneighborhood of 300-550 C. Tubes of the projection type are sometimesmade by securing a conical portion of the envelope to a flat plate ofoptical glass which constitutes the end wall of the tube on which thefluorescent screen is formed and the mask may be applied to such endpiece either before or after the sealing or securing of the end plate tothe conical portion of the tube. If the mask is applied prior to thesecuring of the end plate to the conical portion the nature of thematerial of the mask must be such as to withstand the temperaturesincidental to the securing operation. Suitable materials for thispurpose include opaque vitreous enamels and metallic suspensions knownunder the trade name of Liquid Silver. In such cases the mask may bepainted or otherwise applied to the end plate and thereafter such endplate is sealed to the conical portion of the tube. Alternatively, themask may be applied after the end plate is sealed to the body portion ofthe tube and at the same time as the application of the metallic thoseused in the manufacture of mirrors, or, in-

stead of employing a silverlng solution other chemical methods ofdepositing conductive coatings may be used, for example, a conductingcoating of lead sulphide or copper may be employed, or, alternatively,the required coating may be obtained by the evaporation or sputtering ofmetals in vacuum. One or other of these methods may be employed forcoating the inner wall of the tube andthe end plate and then theconducting layer soformed may be removed from that area of the end plateon which the fluorescent screen is to be deposited, leaving a sharplydefined surrounding mask. 7

In some cases it may be difiicult or laborious to remove the appliedcoating from the end of the tube and to avoid such an operation the areaof the tube on which the fluorescent screen is subsequently formed maybe covered during the coating operation with a sheet or plate ofsuitable material that can be readily removed after the coatingoperation, removal of the sheet or plate likewise removing the coatingapplied thereto during the coating operation, leaving the area of theend of the tube free for the application of the fluorescent screen. Forexample, a sheet of flexible material cut to the required shape andcoated on one side Witha thin layer of low melting point wax or similarmaterial can be readily and firmly fixed into the required position bymelting the wax and allowing it to solidify. In order to remove thesheet the wax is again melted, the sheet removed and benzene or othersolvents thereafter used, to remove any wax adhering to the surface ofthe glass, or in some cases the sheet of flexible material may itself beformed of a material soluble in a suitable medium. Alternatively, anitrocellulose or other suitable lacquer may be applied to the surfaceof the glass prior to the depositing operation, :such lacquer beingsubsequently removed in any known or suitable manner.

In some cases the area on Which the fluorescent screen is to be formedmay be defined by arranging that the surface of the screen be slightlylower or higher than-the surrounding glass to facilitate the applicationof the protecting coating or material particularly in the case where theprotective coating'is initially in liquid form, the liquid conforming insuch cases more readily to the limits required. 7

Where the conducting coating is employed as a mask and is formed from aliquid the required mask or the: area of the screen can be definedwithout the use of a protective layer, if the tube during theapplication of the conducting coating is held at suchan angle, forexample about thatthe liquid is raised to such a level that one side ofthe mask is formed, the liquid level being then reduced and the tuberotated about its axis for 90 and the liquid level again increased tothe required level so that another side of the mask is formed and so on.

In a modification of the invention the fluorescent screen is formed on aplate suspended within the envelope of the cathode ray tube the platebeing made of some transparent material such as glass or mica or of someopaque and preferably conductive material such as aluminum. Where thescreen is transparent, the screen is Viewed from the opposite side tothat bombarded by the cathode ray beam, whilst in the case where thescreen is opaque, it will be viewed from the same side as the sidebombarded by the beam. In both cases the mask is applied to the plate sothat it is co-planar with the fluorescent screen, the mask being appliedbefore or after the plate is fixed within the envelope of the tube. i

The shape of the mask will in most cases be of rectangular shape butother configurations may be employed if required.

What We claim is:

1. The method of preparing luminescent screens for use in cathode raytubes, which comprises the steps of depositing upon a support surface apredetermined thickness of luminescent material to cover a predeterminedarea having a predetermined boundary, depositing a layer of opaquemasking material upon the supporting surface contiguouswiththepredetermined boundary of the deposited luminescent material, andlimiting the thickness of the deposited layer of opaque masking materialto equal substantially the same thickness of the luminescent material.

2. The method of preparing screens for use in cathode raytubes, whichcomprises the steps of affixing a sheet of masking material ofpredetermined configuration to a support surface, depositing a layer ofpredetermined thickness of opaque material upon the support surfacecontiguously with the aflixed sheet, subsequently removing the aflixedsheet, and depositing luminescent material upon the support surfacepreviously occupied by the removed sheet, and limiting the thickness ofthe luminescent material to'substantially equal the predeterminedthickness of the opaque material.

3. The method of Jpreparing luminescent screens for use in cathode raytubes, which comprises the steps of providing a supporting surface,

lowering a predetermined portion of the provided surface, depositingupon the unlowered portion of the surface a'layer of opaque material ofpredetermined thickness, and depositing upon the lowered portion of thesurface luminescent material up to the level of the deposited opaque,material whereby a continuous planar surface of opaque and luminescentmaterial is formed. 7

JOHN WILLIAM STRANGE.

WILLIAM HORACE CONNELL.

luminescent H

